System for estimating bladder volume

ABSTRACT

A system for estimating the volume of fluid in the bladder sequentially scans the bladder with ultrasonic beams that section the bladder into a number of transverse planes. The system determines, from the scan lines associated with a given plane, a plurality of points on each of the front and back walls of the bladder. It then fits a curve to the set of front wall points and another curve to the set of back wall points, to determine an outline of the bladder in the plane. The system next calculates the cross sectional area of the bladder in the plane based on the two curves. After determining the area in each of the planes, the system determines the volume of the bladder by summing weighted version of the planar areas. The system includes a transducer that consists of a plurality of piezo-electric elements held in a relatively thin elastomeric pad and/or substrate that is acoustically impedance matched to skin. The elements are spaced and grouped such that the acoustic beams they produce lie on the plurality of planes, with the first and the last planes separated by approximately seventy degrees. The estimated volume may be is compared with a predetermined overflow threshold, to determine if the user should then void. The system may include a timer that controls the times at which the system produces estimates. The time may decrease the time between estimate calculations if the estimated volume exceeds a predetermined percentage of the overflow threshold.

FIELD OF INVENTION

This invention relates to systems for non-invasively monitoring thevolume of urine in the bladder.

BACKGROUND OF THE INVENTION

Information about the volume of urine in the bladder is importantclinically for several types of patients suffering from bladderdysfunctions. For patients with neurogenic bladders, in whom the nervoussystem connections between the brain and the urinary bladder have beensevered due to spinal cord injury or other disease conditions,sensations that normally alert individuals to the need for voiding asubstantially filled bladder are either significantly reduced or absentaltogether. Such patients are thus at risk of bladder overdistention,which is a condition that, if left untreated, can lead to permanentkidney damage and subsequent renal failure.

For patients with benign prostatic hyperplasia, which results in anenlarged or congested prostate, urine may be retained in the bladderafter voiding. In a substantial percentage of such cases, the prostategrowth constricts the urethra and thus restricts the flow of urine fromthe bladder. In response, the bladder grows thicker and stronger, tocompensate for the increased resistance. Eventually, however, thebladder is no longer able to fully overcome the resistance and emptyingbecomes incomplete.

The retention of urine in the bladder puts the patient at risk ofinfection and bladder overdistention. Bacteria present in the urinarytract tend to multiply in the retained urine, and urinary infection canoccur. The infection, in turn, can worsen the swelling already presentin the prostate, and in the long run can lead to bladder stones orpermanent kidney damage. The amount by which the prostate is enlargedcorrelates poorly with the degree of obstruction that the prostatepresents to the flow of urine through the urethra. It is thereforeimportant that patients with benign prostatic hyperplasia be monitoredto determine the amount of retained urine after every voiding attempt.

Information about the volume of urine in the bladder is important alsofor individuals afflicted with urinary incontinence, especially overflowincontinence. Urinary incontinence is characterized by an involuntaryloss of urine of sufficient quantity and/or frequency to become a socialand health problem. There are two broad classes of incontinence, namely,transient and established. Transient incontinence, which is commonlycaused by an acute illness or the administration of certain therapeuticdrugs, is generally cured by treating the condition that caused theincontinence.

Established incontinence, on the other hand, is chronic and is generallycaused by abnormalities in the function of the detrusor, the bladderoutlet, or both. In many of these patients, the probability of a voidingaccident increases sharply when the volume of urine in the bladdercrosses an overflow threshold. While this threshold may vary frompatient to patient, it is a widely held clinical belief that thereexists a minimum overflow threshold that can be used as an effectivepredictor of such an accident in essentially every patient.Consequently, knowledge that the urine volume has crossed such a minimumthreshold is useful in preventing leakage accidents.

Another class of patients for whom bladder volume information isimportant is patients undergoing lengthy surgery. In such patients,overdistention may occur as a result of anesthesia induced paralysis.Currently, these patients routinely have their bladders intubated within-dwelling Foley catheters. The catheters are used to assess urineoutput and thereby monitor renal function and prevent bladder overdistention. In-dwelling catheters, however, are major sources ofinfection. Furthermore, their use in urine output monitoring is laborintensive and cumbersome. Hence the use of a non-invasive and automaticprocess for this function is desirable.

Ultrasonic systems that use information contained in the backscatteredechoes from the bladder region to determine its volume are known. Onesuch system uses a specialized two-dimensional array transducer whoseindividual elements are activated simultaneously in two groups toproduce first a composite transmit beam and then a composite receivebeam. The signal associated with this composite receive beam isprocessed to extract three dimensional information about the bladder,which in turn is used to calculate its volume. To produce these twocomposite beams, the transducer has to be operated as a phased array;making it complex to manufacture, expensive and too bulky to be worncontinuously.

Another known system uses an ultrasonic transducer that produces asingle beam of ultrasonic energy. The signal received by the transducercontains information about the energy reflected from the back wall ofthe bladder, and the system uses an empirically developed calibrationcurve to indirectly relate this information to the degree of bladderdistention. Consequently, the system produces only relative estimates ofthe bladder distention, rather than estimates of the bladder volume.

Yet another known system produces ultrasound measurements along twoorthogonal planes that are each essentially normal to the direction ofthe patient's spine. Based on the measurements and an ellipsoidal modelof the bladder, the system determines an estimate of bladder volume. Theapparatus requires an operator to position and manipulate the associatedtransducer in a particular way in order to obtain the ultrasonicmeasurements, and thus, the system determines the bladder volume on anevent-by-event basis. Accordingly, this system is not capable ofautomatically and/or essentially continuously producing the measurementsnecessary to produce estimates of bladder volume.

SUMMARY OF THE INVENTION

The invention is a monitoring system that sequentially scans a bladderwith ultrasonic beams that section the bladder into a plurality oftransverse planes. The system determines, from the scan lines associatedwith a given plane, a plurality of points on each of the front and backwalls of the bladder. It then fits a curve to the set of front wallpoints and another curve to the set of back wall points, to determine anoutline of the bladder in the plane. The system next calculates thecross sectional area of the bladder in the plane based on the twocurves. After determining the area in each of the planes, the systemdetermines the volume of the bladder essentially by summing weightedversions of the planar areas.

The system includes a transducer that consists of a plurality ofpiezo-electric elements held in a relatively thin elastomeric pad and/orsubstrate that is applied to the user's lower abdomen. The elements arespaced and grouped such that the acoustic beams they produce lie on theplurality of planes, with the first and the last planes separated byapproximately seventy degrees.

The spacing and grouping of the piezo-electric elements is fixed, andthus, the planar areas can be combined, after each is multiplied by anappropriate design constant that is based on the angles of separation ofthe planes. The estimated volume may be compared with a predeterminedthreshold, to determine if the user should then void.

The transverse planes have been selected such that the associatedmeasurements cover the portion of the bladder that expands in responseto the volume of urine retained in the bladder. While the bladder mayalso expand outside of the region covered by the planes, such expansionis generally not large enough relative to the expansion within themeasured region to render the volume estimate inaccurate for the purposeof determining clinically relevant bladder volumes.

The transducer elements are sized to provide divergent beams, when theyare operated at their resonant frequency. Accordingly, the precisepositioning of the elements on the body is not critical, and thetransducer need not be repositioned for each monitoring procedure. Theelastomer that houses the elements is impedance matched with the skin,and thus, ultrasound gel or a gel substitute is not required. At theinterface with the skin the beams refract, such that the beams can belaunched at angles greater than the tilt of the respective individualelements. The substrate in which the elements are mounted can thus bemade relatively thin, so that the transducer can be continuously worn bythe user. This is in contrast to prior known systems, which usetransducers that are too bulky to be worn comfortably and unobtrusivelyby most users.

The system may automatically monitor the volume of urine in the bladderover time under the control of a timer. Further, the system may retainvolume estimates calculated over predetermined periods of time, toprovide patient histories from which, for example, incontinence patternscan be developed.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention description below refers to the accompanying drawings, ofwhich:

FIG. 1 is a functional block diagram of a system constructed inaccordance with the invention;

FIG. 2 is a schematic representation of a bladder signature in anA-line;

FIG. 3 is a functional block diagram of a processor in the system ofFIG. 1.

FIG. 4 is a diagram of a section of a bladder partitioned into a planeby a set of acoustic beams;

FIG. 5 depicts a transducer included in the system of FIG. 1; and

FIG. 6 is a functional block diagram of the system of FIG. 1 withoptional components.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

Referring to FIG. 1, a monitoring system 10 for estimating the volume ofurine in the bladder includes a transducer 12 that transmits ultrasonicbeams into the bladder region of a patient 15. The transducer, which isdiscussed in more detail below with reference to FIG. 5, includes aplurality of piezo-electric elements 13 that are grouped and spaced toproduce beams that span each of a plurality of transverse planes 16,which lie within a predetermined region of the bladder 14. Thetransducer 12 produces at least "NP" beams, with at least N beamsspanning each of P transverse planes.

The transducer 12 also receives echoes of the tissue structureencountered by the beams, or what are commonly referred to as "A-lines,"and supplies corresponding signals to a transceiver 18. The transceiver18 amplifies and demodulates these signals in a conventional manner andsupplies them to a digitizer 20, which produces corresponding digitalsignals also in a conventional manner. The digitizer 20 then providesthe digital signals to a processor 18 which, as discussed below withreference to FIGS. 2-4, calculates an estimate of the volume of urinecontained in the bladder based on the locations of the bladder walls ineach of the planes.

The ultrasonic beams produced by the transducer 12 are partiallyreflected by the skin, the front wall of the bladder and the back wallof the bladder, each of which is at a different distance from thetransducer. The beams pass with little or no reflection through thefluid contained in the bladder. The bladder thus produces a recognizablepattern, or signature, in the A-line, since the bladder is the onlyfluid-filled region in that part of the body. The pattern thatcorresponds to the bladder is depicted in FIG. 2. The A-line signalsproduced by the transducer 12 are "noisy," however, and the changes inamplitude that represent the bladder walls difficult to discern. Thesystem thus processes these signals to remove the noise.

Referring now also to FIG. 2, the processor 18 determines the locationsof the front and back walls 22 and 24 of the bladder in each plane 16based on the associated A-line signals 30, which are now in digitalform. As discussed below, the processor filters each A-line signal, toremove the noise and enhance rising and falling edges that signify thebladder walls. The processor then determines the location of the frontand back walls in each A-line, by locating the appropriate edges, whichare now peaks in the filtered digital signal.

The processor thus determines N points 20 on the front wall 22 and Npoints 21 on back wall 24 in each plane. The processor 18 nextdetermines a first curve 28 that best fits the N points 20 on the frontwall and a second curve 30 that best fits the N points 21 on the backwall. The two curves define a cross section of the bladder in the plane,and the processor next calculates the cross-sectional area of thebladder in the plane.

After determining the cross-sectional areas in the P planes, theprocessor 18 calculates an estimate of the volume of urine in thebladder, by weighting and combining the P areas. The processor may thencompare the estimated volume with a predetermined minimum threshold forvoiding, and set off an alarm if the volume meets or exceeds thethreshold. The operations of the processor 18 are discussed in even moredetail with reference to FIGS. 3-4 and 6 below.

Referring now to FIG. 3, the processor 18 receives the digital A-linesignal and filters the signal in a filter 40 that includes one or moreof a median filter 42, an averaging filter 44 and an edge enhancingfilter 46. We discuss below a system that includes all three filters.

The A-line signal is first supplied to the median filter 42, whichoperates in a conventional manner to remove noise spikes from the signaland reduce somewhat the various other excursions in the signal that areattributable to the noise. In the system of FIG. 3, the median filter 42is a five-point filter. The filter operates by sliding a five-pointwindow across the digitized data associated with a given A-line. If, forexample, an A-line is represented by "M" digitized points, the filterpositions the five-point window over the first five data points andselects the median of the five data points within the window as themedian filter output value for the i^(th) data point, where i=3 in theexample. The system then replaces the i=3 data point with the medianfilter output value. The filter next slides the window over by one datapoint, to determine a median filter output value for the i=4 data point.The filter repeats this process for the i^(th) data points, where i=5, 6. . ., M-3.

The width of the filter window may be set to a larger or smaller value.Alternatively, the width of the filter window may be set adaptively as asmall percentage of the distance between the front and back walls of thebladder calculated from previous A-line data. The calculation of thedistance "d" between the bladder walls is discussed below.

The output values produced by the median filter 42 are supplied to anaveraging filter 44. The averaging filter operates in a conventionalmanner to smooth the data and further reduce the adverse effects ofnoise in the signal. In the system of FIG. 3, the averaging filter 44 isan eleven-point filter. The averaging filter thus slides an eleven-pointwindow across the data and replaces the digital value of the i^(th)point with the average value of the i-5^(th) through i+5^(th) points fori=6, 7 . . . M-5. By including the median filter before the averagingfilter, spike noise is eliminated. This, in turn, reduces the biasingthat would otherwise be introduced by the averaging filter. A smaller orgreater number of points may be used in the averaging filter window.Alternatively, the width of the filter window may be set adaptively as asmall percentage of the distance, d, between the front and back walls ofthe bladder calculated from previous A-line data.

The output values produced by the averaging filter 44 are supplied tothe edge enhancing filter 46, which manipulates the signal in aconventional manner in accordance with a derivative-based operator. Inthe system of FIG. 3, the edge enhancing filter consists of athree-point kernel [0, 6,-1] that slides across the data. For the i^(th)data point, the filter produces a filter output value that is the sum ofthe i-1^(st), and i^(th) and i+1^(st) data points multiplied by therespective elements of the kernal. Specifically, the sum of the i-1^(st)data point multiplied by 0, the i^(th) data point multiplied by 6 andthe i+1^(st) data point multiplied by -1. The filter replaces the i=2,3, . . . M-2 data points with the corresponding filter output values. Agreater or smaller number of points may be used in the edge enhancingfilter, with corresponding changes made to the kernal.

The filtered signal is next supplied to a wall sub-processor 48, whichdetermines the locations of the front and back walls of the bladder bydetermining, respectively, the locations of associated peaks in thefiltered, digital signal. The portion of the signal that corresponds tothe first 2.5 cm of the body includes reflections from the skin thatgenerally saturate the receiver, and the wall sub-processor 48 thusignores that portion of the signal. Each digital signal value, orsample, is associated with a location index that is converted tocentimeters by multiplying the index by a conversion constant "v," whichis based on the velocity of sound. Accordingly, the wall sub-processorignores the first 50 digital values.

The sub-processor determines the location "L_(F) " of the front wall byfinding the location of an appropriate peak in the remaining signal. Todo this, the sub-processor 48 calculates the difference between thefirst two digital values in the remaining signal and compares thedifference value with a predetermined threshold. If the differenceexceeds the threshold, the processor selects for the front wall thelocation of the first of the two associated digital values, that is, thepoint that is closer to the transducer. If the difference does notexceed the threshold, the sub-processor calculates the differencebetween the digital values associated with the second of the signalsamples used in the previous difference calculation and the digitalvalue associated with the next signal sample, compares the difference tothe threshold, and so forth, until the front wall is located.

Once the front wall is located, the sub-processor searches for the backwall. To determine the location "L_(B) " of the back wall in the A-linesignal, the sub-processor searches the remaining signal for any peakthat exceeds a predetermined second threshold. The sub-processor thuscalculates the difference between each set of two adjacent digitalvalues and compares the difference values with the second threshold. Ifmore than one peak is found, the system determines if the differencevalue associated with one of the peaks is at least 3 times greater thaneach of the difference values associated with the other detected peaks.If so, the sub-processor selects as the back wall location the pointthat corresponds to the first of the two digital values associated withthe greatest difference. If only two peaks are found and the associateddifference values differ by less than a factor of 3, the sub-processorselects the peak that is farther from the transducer. If three or morepeaks are found and associated difference values differ by less than afactor of three, the wall sub-processor identifies a "cluster," which isa set two or more peaks that are located within 10 signal samples ofeach other. The sub-processor then selects as the location of the backwall the position of the median peak within the cluster.

The wall sub-processor also determines the distance "d" in centimetersbetween the front and back walls as: d=(L_(B) -L_(F)) * v, where "*"represents multiplication. The sub-processor 46 supplies the locationindices L_(F) and L_(B) and the corresponding distance d to acurve-fitting sub-processor 50. The curve fitting sub-processor holdsthe values until each A-line associated with a given plane has beenprocessed.

The sub-processor 50 determines if the location of the back wall iswithin a 4.5 cm "dead zone" following the front wall location. The deadzone corresponds to a bladder that contains less than 40 ccs of urine,which is less than a clinically relevant, minimum volume. If the backwall location falls within the dead zone in each A-line associated withthe plane, the sub-processor does not calculate an area measurement. Ifthe back wall location falls within the dead zone in all of the planes,the sub-processor does not calculate the area in any plane, and thus,the system does not calculate a volume is estimate at this time.

In the event that the sub-processor 50 determines that only a relativelysmall number of backwall points fall outside of the dead zone, and thatthere are thus not enough points to compute the volume, sub-processor 50instructs the system to produce another set of A-lines and attempt tolocate the back wall in these A-lines. The system repeats the process atotal of three times, as necessary. If, after three attempts, the systemstill does not have a sufficient number of back wall points, the systemsounds or excites an alarm that indicates that the transducer requiresrepositioning.

Once all of the A-lines have been processed for a particular plane, thecurve-fitting sub-processor 50 determines the cross-sectional outline ofthe bladder in the plane. The processor determines a first curve thatbest fits the set of N front wall points and a second curve that bestfits the set of N back wall points.

Referring now also to FIG. 4, the sub-processor 50 determines x and ycoordinates of the front and back wall points relative to an X axis thatis a line E--E, which extends through the piezo-electric elements E_(l). . . E_(N), and a Y axis, which is a line Y--Y that is perpendicular tothe line E--E and extends through the mid-point of the line E--E. Thesub-processor determines a mid-point between the front wall and backwall locations 42 and 44 in a first A-line 40 that corresponds to thefirst piezo-electric element E₁ associated with the plane. It thendetermines the equation of a line H--H that intersects the A-line 40 atthe mid point and is parallel to the line E--E.

The sub-processor 50 next determines the equation S_(B) of a curvethrough the back wall points that are above line H--H, by determiningthe coefficients of a degree "q" polynomial, S_(B) .tbd.Y=Σα_(j) x^(j)for j=0, 1 . . . q where in this system q=N-1, and N is the number ofA-lines that span the plane. Following the method of least squares, thesystem solves the following set of normal equations to determine thecoefficients a_(j) : for i=1, 2 . . . N-1 ##EQU1## where x_(k) and y_(k)are the x and y coordinates of the known points on the back wall.

The system similarly determines the equation S_(F) of a curve throughthe known points on the front wall that are below the line H--H. It nextdetermines the points U and V where the curves S_(F) and S_(B) interceptthe line H--H. These points are the bounds of integration to determinethe area A_(B) below the curve S_(B), and the area A_(F) below the curveS_(F). The area bounded or enclosed by the curves S_(F) and S_(B), whichis A_(B) -A_(L), is the cross-sectional area of the bladder in theplane.

To determine the area A_(B) below the curve S_(B), the system usesSimpson's Rule for degree q polynomials and numerically solves theequation: ##EQU2## The sub-processor similarly determines the area A_(F)below the curve S_(F), and then subtracts the two areas to produce theestimated area of the bladder in the plane.

The sub-processor then calculates the cross-sectional area in each ofthe remaining P planes.

Once the areas in all P planes are calculated, the sub-processor sumsthem, after weighting each one by an associated design constant D_(P),for p=1, 2 . . . . P. The design constant is associated with the anglebetween the p^(th) plane and a first, essentially horizontal, plane. Thesum is an estimate of the volume in the bladder.

Referring now to FIG. 5, the transducer 12 includes at least NPpiezo-electric elements 13. The elements are grouped and spaced toproduce at least N beams in each of the P planes, with N beams spanningthe plane. As discussed above, the last of the P planes forms a 70°angle with the first of the P planes. A controller (not shown) operatesthe elements 13 in an appropriate order. Preferably, the elements areoperated sequentially to produce in order all of the beams associatedwith the first plane, then the beams associated with a next plane, andso forth.

The piezo-electric elements 13 are dimensioned such that they producedivergent beams at their resonant frequency. Each element 13 thusproduces a cone-shaped beam pattern for the transmission of theultrasonic beams and the reception of the echoes. Consequently, eachelement is capable of responding to acoustic energy traveling along anyray that lies within the associated cone-shaped region. It is thustolerant of angular displacement that lies within a range defined by theangle that the cone-shaped beam pattern subtends at the element. Inorder to strike an optimal balance between tolerance to angularorientation and the level of "noise" introduced into the signal, thedimensions of the element are selected such that the associatedcone-shaped beam profiles subtend an angle, at the element, that is inan approximate range of 5° to 15°.

The piezo-electric elements 13 are mounted in a relatively thin padand/or substrate 50. The elements are completely encased in anelastomeric material such that the surfaces that come in contact withthe patient's skin are soft and conform to the contours of the abdominalregion. Further, the elastomeric material is chosen so that it providesacoustic impedance matching with the skin, in order to minimize acousticreverberations at the skin/transducer interface. There is thus no needfor ultrasound gel, or a gel substitute, as is typically required withknown prior devices.

Additionally, the elastomeric material is chosen such that the beamsproduced by the elements refract in a predetermined manner away from thenormal at the transducer/skin interface. Accordingly, the actualphysical tilt of the elements necessary to direct the acoustic beamstowards the bladder are significantly smaller than with known priordevices. This allows the transducer 12 to be significantly thinner inprofile than the known prior transducers, and the transducer 12 can thusbe worn more conveniently and comfortably by a user.

Referring now to FIG. 6, the system 10 may include a display device 60that displays and/or stores the volume estimate. The system may also, orinstead, include an alarm sub-system 62 that compares the volumeestimate with a predetermined minimum voiding threshold, and sets off analarm 63 if the estimate meets or exceeds the threshold. The alarm,which notifies the user of the need to void, may be a visible, audibleor vibrational alarm, as desired. The system preferably repeats itsmonitoring operations at predetermined time intervals, under the controlof a timer 64. Once the estimated volume exceeds 75% of thepre-established minimum threshold value, the system increases thefrequency with which the monitoring operation is performed. The systemmay also retain the estimates, to provide a patient history.

The foregoing description has been limited to a specific embodiment ofthis invention. It will be apparent, however, that variations andmodifications may be made to the invention, with the attainment of someor all of its advantages. For example, the system may be used toestimate the volume of fluid in any fluid-filled organ that issurrounded by soft tissue. Further, the various processors andsub-processors may be hardware, software or firmware. Therefore, it isthe object of the appended claims to cover all such variations andmodifications as come within the true spirit and scope of the invention.

What is claimed is:
 1. A system for estimating the volume of fluid in anorgan, the system including:A. a transducer fori. providing to a regionof a body in which the organ is situated a plurality of ultrasonic beamsthat section the organ into a plurality of transverse planes that areangularly offset from a first transverse plane and section eachtransverse plane into a plurality of subsections; ii. receiving from theregion echoes that correspond to the beams, and iii. producing A-linesignals that are associated with the echoes; B. means for determining ineach of the plurality of A-lines associated with a given plane locationsof front and back walls of the organ; C. means for determining anoutline of the organ in the given plane based on the locations of thefront and back walls in the plurality of A-lines associated with theplane; D. means for calculating a cross-sectional area of the organ inthe given plane based on the outline of organ in the plane; and E. meansfor determining an estimate of the volume of fluid as the weighted sumof the cross-sectional areas calculated for the planes, with the weightassigned to a given area being associated with the angle thecorresponding plane makes with an approximately horizontal plane.
 2. Thesystem of claim 1 wherein the transducer produces at least NP beams,where P is the number of planes and N is a number of beams that span agiven plane.
 3. The system of claim 1 wherein the transducer producesbeams that span planes that form an angle of approximately 70° between alast plane and the approximately horizontal plane.
 4. The system ofclaim 1 wherein the means for determining locations of front and backwalls in the A-line signals includes one or more filters for filteringnoise from the signal and enhancing edges in the signal.
 5. The systemof claim 1 wherein the means for determining an outline of the organfits a first curve to the front wall locations associated with the givenplane and a second curve to the back wall locations associated with theplane.
 6. The system of claim 5 wherein the means for calculating anarea of the organ uses Simpson's Rule to determine in the given planethe area under the second curve that is associated with the back walllocations and the area under the first curve that is associated with thefront wall locations, and subtracts the two areas to calculate thecross-sectional area of the organ in the plane.
 7. The system of claim 1wherein the transducer remains in a given position and sequentiallyproduces the plurality of ultrasonic beams associated with a givenplane, followed by the plurality of ultrasonic beams associated with anext plane, and the plurality of ultrasonic beams associated with eachsuccessive plane until the beams associated with all of the planes areproduced.
 8. The system of claim 1 wherein the transducer includes:a. aplurality of piezo-electric elements that are arranged to produceultrasonic beams that section the bladder into the plurality of planesand each plane into a plurality of subsections; b. a relatively thin padthat houses the piezo-electric elements, the pad being acousticallyimpedance matched to skin; and c. means for controlling the operation ofthe piezo-electric elements, to operate them sequentially to produce theultrasonic beams that span and section each of the plurality of planeswithout repositioning the transducer or moving any of the piezo-electricelements.
 9. The system of claim 8 wherein each piezo-electric elementis dimensioned to produce a divergent beam when the element is operatedat a resonant frequency.
 10. The system of claim 1 further including atimer that controls the times at which the system produces theestimates.
 11. The system of claim 10 wherein the timer decreases thetime between estimate calculations if the previous estimate is within apredetermined percentage of an overflow threshold.
 12. The system ofclaim 1 further including an alarm that is energized if the volumeestimate exceeds a predetermined overflow threshold.
 13. The system ofclaim 1 ftirther including means for storing the volume estimates.
 14. Asystem for estimating the volume of fluid in an organ, the systemincluding:A. a transducer that remains in a given position on the body,the transducer providing to a region in which the organ is situated aplurality of ultrasonic beams that section the organ into a firsttransverse plane and a plurality of transverse planes that are angularlyoffset from the first plane and section each transverse plane into aplurality of subsections, and producing corresponding A-line signals,the transducer including:a. a plurality of piezo-electric elements thatare arranged to produce ultrasonic beams that section the bladder intothe transverse planes and each plane into a plurality of subsections; b.a relatively thin pad that houses the piezo-electric elements, the padbeing acoustically impedance matched to skin; c. means for controllingthe operation of the piezo-electric elements, to operate themsequentially to produce the ultrasonic beams associated with each of therespective planes; B. means for determining locations of front and backwalls of the organ in the plurality of A-lines associated with a givenplane; and C. means for calculating a volume estimate based on thelocations of the front and back walls of the organ in each of thetransverse planes.
 15. The system of claim 14 wherein the means forcalculating the volume includes:i. means for determining an outline ofthe organ in the given plane based on the locations of the front andback walls in the plurality of A-lines associated with the plane; ii.means for calculating a cross-sectional area of the organ in the givenplane based on the outline of organ in the plane; and iii. means fordetermining an estimate of the volume of fluid as the weighted sum ofthe areas calculated for the planes, with the weight assigned to a givenarea being associated with the angle the corresponding plane makes withan approximately horizontal plane.
 16. The system of claim 14 whereinthe transducer produces at least NP beams, where P is the number ofplanes and N is a number of beams that span a given plane.
 17. Thesystem of claim 14 wherein the transducer produces beams that spanplanes that form an angle of approximately 70° between a last plane andthe approximately horizontal plane.
 18. The system of claim 14 whereineach piezo-electric element is dimensioned to produce a divergent beamwhen the element is operated at a resonant frequency.
 19. The system ofclaim 14 further including a timer that controls the times at which thesystem produces the estimates.
 20. The system of claim 19 wherein thetimer decreases the time between estimate calculations if the previousestimate is within a predetermined percentage of an overflow threshold.21. The system of claim 14 further including an alarm that is energizedif the volume estimate exceeds a predetermined overflow threshold. 22.The system of claim 14 further including means for storing the volumeestimates.